finding speed

I'm almost 4 weeks post marathon. I tried to some pick-ups yesterday and what was just a fun, easy thing pre-marathon, was sheer torture. (3 minutes at 5k pace). I obviously have no tolerance whatsoever.

I'm thinking of just going out and running a 5k so I know what torture is, knowing that I will “gut it out” in a race. Anyone try this sneak attack. thoughts ???